1. Field of the Invention
The present invention relates to a fuel cut-off device for an internal combustion engine. The fuel cut-off device is used to protect the engine against the effects of excessive rotational speed (rpm rate). The engine preferably has an electronically controlled fuel injection device and is mounted in an automobile.
2. Description of the Related Art
The use of an electronically controlled fuel injection device to supply fuel to a combustion chamber of an internal combustion engine mounted in an automobile is well known. Such an electronically controlled fuel injection device comprises a fuel injector arranged for each cylinder of the engine at an intake manifold and a control device to control the timing and the length of opening of the fuel injector so as to provide an optimum air-fuel ratio in response to sensed engine operating conditions.
Namely, in this electronically controlled fuel injection device, a basic fuel injection time period is calculated in response to basic operatrng conditions of the engine, based on an engine load generally represented by an intake air volume into the engine or an intake air pressure at the air intake manifold and a rotational speed of the engine, then correction factors based on signals from several sensors arranged on the engine are added to the basic fuel injection time period to provide a final fuel injection time period. Thus, at each revolution of the engine, the fuel injector is opened by the above calculated time period to supply fuel to the combustion chamber.
A fuel cut-off is usually effected by prohibiting the fuel injector from opening at the above opening timing. There are two types of generally established fuel cut-off operations. In one method, a fuel cut-off is carried out during deceleration of the engine at a relatively low engine rotational speed (for example, 1300-900 rpm). This fuel cut-off method improves fuel economy, reduces unburned hydrocarbon components in the exhaust gas, and prevents the catalytic converter from overheating. In another method, the fuel cut-off is carried out at an extremely high engine rotational speed (for example, 6500 rpm), which the engine rarely attains. Therefore, this latter type of fuel cut-off rs rarely effected if the automobile is driven in a normal manner. However, the engine rotational speed may reach such an extreme rate if the automobile is driven in an aggressive manner. It is from this viewpoint that a fuel cut-off is carried out, to protect the engine from such an abnormal rpm rate.
Conventionally, a fuel cut-off operation to protect the engine from excessive rotational rate is effected when the engine rotational speed exceeds a constant reference speed, not only when the engine is running under a load but also when it is running under a "no-load" "racing" condition. Note, the term .differential.racing" in this context is generally understood in the art to refer to an engine operating condition in which the engine rotational speed is increased under a "no-load" state. This state often occurs when an accelerator pedal is pushed down while the transmission of the automobile is in neutral position and the automobile is stopped or stopping, and sometimes occurs when the automobile is running with the transmission in neutral position.
With the above type of fuel cut-of control, the driver can feel that the fuel cut-off operation is effected when the engine is operating under a load condition and the automobile is running, because the automobile does not accelerate. However, if the fuel cut-off is carried out in the racing condition and the car is stopping, the driver may not feel the effect of the fuel cut-off operation. Thus the driver may continue to keep the engine in excessive speed, and the fuel supply and the fuel cut-off operations may be repeated many times over a long period of time. This leads to an increase in the vibration and noise of the engine, and the engine is subjected to extreme thermal conditions over a correspondingly long period of time.
Japanese Patent Application No. 58-238220, filed on Dec. 16, 1983, by the same applicant (assignee) as for the present application, relates to a fuel cut-off method intended to protect the engine from such a high rotational speed. The above application discloses the steps of: detecting whether or not the rotational speed of the engine exceeds a fuel cut-off reference value; cutting the supply of fuel if the rotational speed of the engine exceeds the fuel cut-off reference value; and gradually reducing the fuel cut-off reference value to a lower limit determined as being above the normal-use engine rotational speed range, if the fuel cut-off operation is continuously effected. The concept of the above application is similar in principle to that of the present invention, i.e., to protect the engine from an abnormal rpm rate. However, the previous application does not include the concept of changing the fuel cut-off reference speed between the engine load condition and the engine racing condition.